How do I find the last seen date for my iPhone device?

I assume you me for a Lost phone. First you would have to had Find My enabled in order to view any location. You can use this Support Article here for more information and I highlighted a couple of points that may apply.

• If Find My can't display your device or item's current location, for example if your device's battery has run out, you can see its last known location for up to seven days in the Find My app.
• If more than seven days have passed since your device last sent its location to Apple through the Find My network, Find My can't display a location. Instead it shows "No location found."

Use Find My to locate your lost Apple device or AirTag - Apple Support

If you mean something else, you will need to provide additional information.

In addition to the excellent suggestions already supplied:

Apple support's YouTube video (6:32 min) may be helpful:

How to use Find My on iPhone, iPad, and iPod touch | Apple Support

https://www.youtube.com/watch?v=qj9ORN8IXXk

and /or

Apple Support's online article about lost or stolen devices provides many details:

See what to do: If your iPhone or iPad was stolen - Apple Support